An evaporation fuel processor has been proposed having a canister into which evaporative fuel generated in a fuel tank is temporarily adsorbed. As the need arises, the evaporative fuel is purged from the canister to an intake air passage of an engine.
For instance, Japanese Patent Publication No. 05-018326 and Japanese Patent Publication No. 06-101534 each disclose such an evaporative fuel treatment system. The system calculates an evaporative fuel concentration in a mixture to be purged to the intake air passage prior to purging. More specifically, the system detects a flow rate or a density of an air-fuel mixture in a passage through which the air-fuel mixture is purged to the intake air passage. Furthermore, the system detects a flow rate or a density of air in a passage open to the atmosphere. The system calculates the evaporative fuel concentration from a ratio of these detection results.
In this conventional system, a negative pressure of the intake air passage is applied to each passage, whereby the mixture or air flows through each passage and the flow rate or density is detected. With this configuration, when pulsation occurs in the negative pressure of the intake air passage, a fluctuation will arise in the flow rate or density. Therefore, calculation of the evaporative fuel concentration based on the flow rate or density becomes less accurate. Moreover, where the negative pressure of the intake air passage is small, the flow rate of the mixture or air in each passage is also relatively small. Accordingly, detection of the flow rate or density itself can be difficult.
Accordingly, an evaporative fuel treatment system has been proposed that depressurizes a detection passage having a reduced area portion (i.e., orifice, throttle, contraction, reference passage, etc.) with depressurizing means (e.g., a pump). The system detects a cutoff pressure of the detection passage with the atmosphere side of the reduced area portion blocked. The system also detects an air pressure with the reduced area portion and the atmosphere brought into communication. Furthermore, the system detects a mixture pressure of the mixture with the reduced area portion and the canister being brought into communication as a differential pressure at both sides of the reduced area portion. The system calculates the evaporative fuel concentration based on the cutoff pressure, the air pressure, and the mixture pressure. Since the detection passage is depressurized by a pump, the differential pressure of a detection object is stabilized if there is substantially no change in detecting conditions, and the flow rate of air or mixture is sufficiently maintained in the detection passage.
In contrast to this, an evaporative fuel treatment system illustrated in FIG. 19 detects the cutoff pressure Pt, the air pressure ΔPAIR, and the mixture pressure ΔPGAS on a continual basis. In addition, until detection of these three kinds of pressures is completed, a purge valve is closed and purging the evaporative fuel is not performed. As a result, a substantial amount of time may elapse during which the purge cannot be performed due to detection of the three kinds of pressures. In another case, when the three kinds of pressures cannot be detected on a continual basis because a pressure detectable time is short, the purge cannot be performed until the three kinds of pressures are detected. Accordingly, even when the purge execution condition exists, the purge may not occur. This incapability may lead to decrease in the number of purges. Start of the purge may also be delayed, which can cause a shorter purge time, and accordingly the quantity of the purged evaporative fuel may be decreased.